At the beginning of the first day of school there

At the beginning of the first day of school there was a dead boy found inside of a classroom. They found 3 suspects: The janitor, the teacher and one of the boy's classmates. The janitor claims that he was just cleaning the room, the teacher said that she was computing her student's grades and the boy's classmate claims that he was listening to music. Who committed the crime?

The Teacher. It's still the first day of school so there would be no grades.